Artifacts or devices which are intended for use in a living animal body usually are made of materials having limited biocompatibility, such as non-thrombogenicity and compatibility vis-a-vis living tissue or blood. In the instant disclosure the term "animal" refers in particular to mammals including man. Such artifacts are therefore generally coated with surface coatings for improving the biocompatibility, such coating being made without affecting other important properties of the artifact. In recent years such coating of implantable artifacts has been provided by so called plasma polymerization. Such technique has been used mainly when coating artifacts made of polymeric materials. Plasma polymerization generally is based on introducing a gas comprising one or more polymerizable organic monomers into a vacuum zone, wherein the material to be coated is placed. The polymerizable monomers are then subjected to an electric discharge for initiating polymerization reactions by the generation of ions of free radicals reacting with each other and also with the substrate when made of an organic material perform a deposit on the substrate. The polymerizable monomers are often constituted by fluorinated hydrocarbons, such as tetrafluoroethylene.
Although plasma polymerization for modifying biomaterials has been used as early as in the late 1960's relatively little work has been done until recent years, but presently the number of publications and patents is rapidly increasing and plasma polymerization has become increasingly interesting for improving the surface properties of implantable devices or artifacts.
As examples of close prior art there may be mentioned U.S. Pat. Nos. 3,839,743, 4,188,426 and 4,656,083, each relating to technique for improving the biocompatibility of implantable devices or prostheses. However, in regard to this prior art it is evident that the plasma polymerization to provide for biocompatibility is applied solely to non-metallic substrates, whereby adherence of the polymer coating obtained to the underlying substrate is greatly facilitated due to the fact that covalent bonds can be formed between substrate and coating in the plasma polymerization process.
The present invention deals exclusively with substrates made of electro-conductive materials, such as metallic materials, and furthermore, substrates which are hollow and have perforated walls. In particular, the invention is directed to tubular substrates of metallic materials. When trying to solve the problem of coating such electro-conductive substrates using plasma polymerization it will be immediately clear that due to the fact that the substrate acts as a Fareday's cage the plasma polymerization fails to function properly in that inside such cage no generation of plasma will occur and thus no deposition of polymer.
The main object of the present invention is to provide a new method of manufacturing biocompatible hollow artifacts with perforated walls made from a substrate of electro-conductive material by depositing a coating onto said substrate which results in improved biocompatibility at the site of implantation of such prosthesis into a living animal body.
Another object of the invention is to enable manufacture of tubular artifacts with perforated walls made from a substrate of electro-conductive material using plasma gas discharge in a plasma zone containing a plasma polymerizable monomeric gas to form a biocompatible thin flexible coating on said substrate.
Yet another object of the invention is to provide prostheses, the substrates of which are coated with a carbonaceous coating of improved biocompatibility, such as non-thrombogenicity and tissue or blood compatibility.
Still another object of the invention is to provide techniques for coating expandible tubular stents, such as self-expanding ones, with a thin flexible, strongly adhering coating of excellent biocompatibility.